Multiplex system



March 3, 1931. c. H. FETTER 1,794,889

MULTIPLEX SYSTEM Original Filed Dec. 3, 1924 IN VEN TOR.

BY F /yf Q( A TTORNEY Patented Mar. 3, 1931 UNITED STATES CHARLES H.FETTER, OF MILLBURN, NEW JERSEY, ASSIGNOR TO AMERICAN TELE- PATENTOFFICE PHONE AND TELEGRAPH COMPANY, A CORPORATION 0F NEW YORK MULTIPLEXSYSTEM Grignal application 1ed`1December 3, 1924, Serial No. 753,729.Divided and this application filed June 21, 1927. Serial No. 200,419.

This invention relates to multiplex systems, and more particularly, tomultiplex systems employing piezoelectric crystals for selecting thechannels of transmission.

This is a division of applicants copending application, tiled December3, 1924, Serial Number 753,729.

vWhere multiplex transmission is attained by the use of carrier currentsa plurality of carrier frequencies are transmitted over a commontransmission circuit. the carrier frequencies being separatedsufficiently to prevent interference and each frequency being assignedfor use as a separate channel of communication so that signals may betransmitted by impressing them upon the carrier frequencies.

It has long been known that certain crystals have electric propertieswhen under mechanical stress, and these crystals are known aspiezo-electric crystals. rIhose that are the most stronglypiezo-electric are the crystals of quartz and Rochelle salts. A rod cutfrom such a crystal, having its sides connected to a source ofalternating current of the proper frequency, vibrates and reacts uponthe alternating current circuit. It has also been discovered that thesecrystals may be made to act as transmitters (the direct piezo-electriceffect) as Well as receivers (the converse effect).

Piezo-electric crystals have been developed which, when employed inconnection With oscillators, act as frequency stabilizers, therebyrendering the oscillatingcircuit practically free from disturbingcapacity effects, battery voltage variations, etc. One Way in which thiscan be accomplished is by inserting a crystal serially in the platecircuit of au oscillator. When the crystal is connected in this mannerit offers very little resistance to a frequency which is a function ofits physical dimensions, and a comparatively high impedance to otherfrequencies. Therefore, when the electric-al circuit is tuned to thenatural frequency of the crystal, maximum energy is obtained from theoscillator, and when the lfrequency of the electrical circuit` is notthe same as the natural crystal frequency, the frequencv of *beoscillator will remain the same, being governed by the frequency of thecrystal, but the output will be considerably reduced.

Crystals having piezoelectric properties also display characteristicswhich make them useful as resonators, behaving very much like anelectric circuit`having series inductance, capacity and resistance. Theimpedance characteristic of the crystal is such that the resonance curvefor the crystal is very much sharper than a corresponding curve for anelectrical circuit of the type that is Widely in use at the presenttime.

One of the objects of this invention is to obtain the selection of aparticular frequency band in a4 multiplex system by transmitting thefrequencies of all of the channels and then suppressing the undesiredchannels by means of resonators of the piezo-electric type.

Another object of this invention is to provide a system in which aplurality of frequency channels may be employed in a small range of thefrequency spectrum.

This invention, both as to its organization and method of operation, asWell as the further objects, features and advantages thereof, will bemore fully understood by reference to the detailed descriptionhereinafter following When read in connection with the accompanyingdrawing in which Figure l diagrammatically illustrates the useof'piezoelectric crystals as frequency stabilizers; Fig. 2l illustratesa separate oscillator the frequency of which is controlled by crystals;and

Fig. 3 illustrates the use of crystals as ab sorbers in obtainingselectivity between a plurality of channels. The same parts aredesignated by the same reference characters Wherever they occurthroughout the several "'f views.

Referring to Fig. l of the drawing there is represented the incomingportion of a car rier suppression system upon which may be superposed alplurality of channels of different frequency bands. In practice a numberof programs are superposed upon the separate carrier frequencies in amanner Well understood in the art. All of the programs being modulatedupon separate carrier frequencies may be transmitted simultaneously overline 1 as side bandsl of the modulated carrier frequencies. Line 1 isshown as terminating in inductance 2. Signals coming in over line 1 areimpressed upon the inductance 2 and are also impressed upon inductance 3in a manner well understood in the art. Batteries 8, 9 and 10 arefilament, grid and platel batteries respectively. For the purpose ofillustration 1t has been assumed that only three carrier channels, eachrepresenting a side band of the carrier frequency, are superposed uponthe line 1. -Each of the crystals 11, 12 and 13 has a. natural vibrationat the frequencies of the carriers of the separate channels. A capacity15 is interposed serially in the circuit ofthe crystal in order to keepthe plate potential off the grid of the tube. By moving the switch 14 toconnect the crystal corresponding to the carrier frequency of thedesired channel, there will thus be impressed on the input circuit ofthe oscillatoretector` tube 4 two frequencies, one, the side bandsuperposed on line 1, and two, the freuency corresponding to the crystalvibration. The two frequencies beat 1n the input circuit, and asa resultthereof the original signaling frequency appears in the output circuit othe oscillator-detector tube 4, among other much hi 'her frequencies. Bmeans of the coupled lnductances 5 and 6 t e products of vthe beatingreatcion are impressed on line 7, which leads to a low frequencyreceiving set. However, to eliminate the high frequencies a lowfrequency filter may be interposed in the circuit connecting inductance6 and the low frequency receiving set.

Fig. 2 represents a modification of Fig. 41

showinganother arrangement in which the `same results are obtainable 1na carrier supl pression system. In this figure there is a separateoscillator 17 and the frequency thereof is determined by the crystals11, 12 or 13, depending upon which crystal is connected by switch 14.Thus in the input circuit of the oscillator-detector tube 4 there arefound the side bands of line l and the carrier frequency of theparticular program desired as determined by the crystal connected byswitch 14.

As these frequencies beat in the input circuit of theoscillator-detector tube 4 there appears in the output circuit .theoriginal signaling frequency and other much higher frequencies. Again,by using a low pass filter these high frequency components areattenuated and eliminated, and the low frequency or voice frequencycurrents Aare impressed on line v7' from which they are led toanappropriate receiving set.

. Thus for both Figs. 1 and 2 above the incoming signals consist of theplurality of side f bands, and the particular programdesired is obtainedby beating any one of these incom-4 mg side bands with the properfrequency 1o- 'ca y supplied to the oscillator-detector tube.

It is important in the operation of such a system that the localoscillator be very constant in frequency and free from harmonics. Theserequirements are met by employing the crystal as a stabilizer. Anotheradvantage which is particularly noteworthy is that there is no tupingnecessary for the Operation of crysta s.

In Fig. 3 there is illustrated the means for employing crystals tosuppress all undesired, channels. The crystals are arranged forsuppressing programs impre'ssed on linel which are present in the formof a carrier and a side band. Thus one crystal is provided at thereceiving end corresponding to each channel impressed on line 1, andadapted so ast-o be bridged across the line 1 by means ofA switches 19,20 and 21 in any desired combination" The three programs, as assumedmerely vfor the purpose of illustration, modulated -individuallyupon'three' different carrier freing the crystals 11, 12 and 13, tunedto the carrier frequencies transmitted. By closing switches 19 and 21line 1 willbe short-circuited at the frequencies corresponding tocrystals 11`and 13, and the frequency'corquencies, have bridgedlcircuits each -includresponding to crystal 12 will be transmitted I and13 will be effectively suppressed, and by closing switches 19 and 20 thefrequency of crystal 13 will be transmitted and the frequencies of 11and 12 will be effectively suppressed. v

It is to be noted that by bridging a crystal across line 1 as abovedescribed the frequency of that carrier to which the crystal correspondswill be greatly attenuated. The attenuation resulting from bridging eachof the crystals across line 1 is sufficient to effectively suppress thecarrier frequency corresponding to the crystal thus interposed. By thisarrangement all frequencies near the crystal frequency are attenuatedvery distinctly while frequencies further away. from the crystalyfrequency are practically unaffected thereby. However, Fig. 3, theillustration under discussion, represents a carrier transmission systemof the type well understood in the art. In such a system, although oneside band corresponding to the voriginal voicecurrent is alsotransmitted along with `the carrier, yet

by bridging across line 1 the crystal 'having suppressing the carrierfrequency the side band will remain ineffective, and the programcorresponding to the frequency will be nullified to all intents andpurposes. And if two of the carrier frequencies are suppressed bybridging the corresponding crystals across the transmission line,`thethird carrier frequency will be transmitted to the demodulator 4 alongwith its side band and there properly demodulated, while the side'bandsof the suppressed carriers-Will remain ineffective. v

It is possible, with the arrangements of this invention, to use manymore channels in a given frequency range than in former arrangements ofmultiplex systems using tuned electrical circuits, because of the factthat crystals are much more selective than the corresponding tunedelectrical circuits. And the method of selection herein described may beapplied to all kinds of circuits employing carrier currents, although itis particularly applicable to systems for music distribution over lineWires to a plurality of subscribers. It will be obvious that While theinvention has been illustrated in certain particular arrangements, it iscapable of embodiment in many and Widely different arrangements Withoutdeparting from the spirit of the invention or the scope of the appendedclaims.

lVhat is claimed is: l 1. In a program transmission system, a line uponwhich a plurality of different carrier frequency combinations may beimpressed,

- each carrier frequency combination including a carrier Wave and a bandof signals superimposed thereon, a three-element vacuum tube the inputcircuit of which is coupled to said line, andv a. plurality ofpiezo-electric crystals shunting said line and vibrating at naturalperiods corresponding to the carrier Waves of undesired carrierfrequency combinations, each piezo-electric crystal being in mechanicalvibration for the carrier Wave of each undesired carrier frequencycombination and suppressing the effect of said carrier Wave, saidpiezo-electric crystals being mutually independent in their mechanicalvibrations and ynon-reactive, desired frequency combinations and thebands of signals of undesired carrier frequency combinations beingimpressed on the input circuit of said vacuum tube, said vacuum tubedemodulating the de sired carrier frequency combina-tions so that theproducts of demodulation may appear in the output circuit of said vacuumtube, the bands of signals of undesired carrier fre- -quencycombinations being renderedineffective.

2. In a multiplex program transmission system, a line upon which areimpressed a plurality of diffe-rent modulatedcarrier frequencycombinations, each carrier frequency combination including a carrierwave and a band of signals superimposed thereon, selecting meansconsisting of independent and mutually non-reactive piezo-electricdevices associated with said line to suppress the carrier waves ofundesired modulated carrier frequency combinations and to pass freelythe carrier Waves of desired modulated carrier frequency combinations, ademodulator for demodulating the desired carrier frequency combinations,the bands of signals of undesired carrier frequency combinations be ingimpressed upon said demodulator and being rendered ineffective thereby,and means associated With said demodulator for observing the desiredsignals.

8. In a multiplex program transmission system in which a plurality ofcarrier currentA frequencies are each modulated by signals and impressedon a transmission circuit, selective means consisting of a plurality ofpiezoelectric devices associated With the said transmission circuit tosuppress the carrier current frequencies of undesired carrier currentcombinations and to transmit freely all components of desired carriercurrent combinations and the signals previously modulated on the carriercurrent frequencies of undesired carrier frequency combinations, eachpiezo-electric device suppressing the carrier current frequency of eachundesired carrier frequency combination so that the signals modulatedthereon may be rendered ineifective and unsuitable for demodulation, anda demodulator in which only the desired carrier current combinations aredissociated to reproduce the original signals. A

4. In a program transmission system, a circuit upon which are impresseda plurality of closely spaced carrier currents each of which ismodulated by signals, a selective arrangement associated with saidtransmission circuit to transmit only one of the carrier components ofsaid modulated carrier current-s to the exclusion of the others, saidselective arrangement consisting of a plurality of mutually independentand non-reactive mechanical or crystal resonators which vibrate atfrequencies corresponding to the carrier frequecnies of the signalsimpressed upon said transmission circuit, and a demodulator upon whichare impressed the signals correspond` ing to the various carriercurrents untransmitted by said selective arrangement and in which thedesired carrier current andthe signals modulated thereon are demodulatedto reproduce only the desired signals.

5. In a program transmission system in which a plurality of carriercurrents are each modulated by signals and impressed on a transmissioncircuit, a selecting arrangement to suppress the carrier components ofthe undesired modulated carricr currents to the eX- clusion of theothers, said selecting arrangement consisting of a plurality ofpiezo-electric resonators which are independent and mutuallynon-reactive each of which vibrates at a frequency corresponding to acarrier component impressed upon tlie transmission circuit, meansassociated With said selecting arrangement to connect in parallelrelationship 'across the transmission circuit piezoelectric resonatorscorresponding to the signals which are to be suppressed, and a vacuumtube device for demodulating the desired carrier current signal, to thusreproduce the desired signals.

6. In a program .transmission system, a transmission circuit in which aplurality of channels of communication are transmitted, each channelconsisting of a carrier frequency and one side band, selecting meansassociated with said transmission circuit to suppress the carrierfrequencies of all undesired communication channels to the exclusion ofthe others, said selecting means comprising a plurality ofpiezo-electric devices each of Vwhich mechanically vibratesindependently at af frequency corresponding to a. carrier frequency of acommunication channel, means to connect said piezo-electric devicescorres onding to the communication channels to e suppressed in parallelarrangement in said transmission circuit,

andv means consisting of a deinodulator to "dissociate the desiredcommunication channel in order to reproduce the original signals of-said desired communication channel.

" 7. In a signaling system for program transmission in which a pluralityof channels of communication are transmitted and in order to separatedesired signals from the carrier frequency of the channel, the sidebands of undesired channels being rendered substantially ineffective.

8. In a signaling system for program transmission in which a pluralityof carrier frequencies aie each modulated by signals and impressed upona transmission circuit, means for selecting certain carrier frequenciesto the exclusion of the others, said selecting means comprising aplurality of piezoelectric resonators Which are in independentmechanical vibration and mutually non-reactive, each piezo-electricresonator vibrating at the frequency of the carrier of a particularmodulated carrier current,and means for connecting 'saidpiezo-electricresonators messes piezo-electric resonators which are in mechanicalvibration and are mutually independent and non-reactive,` eachpiezo-electric resonator vibratin at the frequency of the carrier of aYmodu atedcarrier current, means for connecting said piezo-electricresonators in parallel relationship across the transmission circuit tosuppress all but one Aof the carrier frequencies of the various carriercurrent combinations, the signals previously modulated yon eachsuppressed carrier frequency being freely transmitted,l

and means consisting of a demodulator associated with said transmissioncircuit for demodulating the signals corresponding to tlie side bands ofthe unsuppressed carrier to obtain the desired signals originallymodulated thereon.

10, In a signaling system for program transmission, a pair ofintercommunicating stations, a plurality of signaling channels at onestation, a pluralityof corresponding receiving channels at the otherstation, a

demodulator at said second lmentioned station, means whereby carriercurrents modulated in accordance with signals are transmitted from eachof said first mentioned channels to each of said second mentionedchannels and applied to the demodulator thereat, and selective meansconsisting of a plurality of piezo-electric devices whereby only thecarrier currents of undesired modulated carrier currents will besuppressed, .theA demodulator separating the signals from theunsuppressed carrier current upon which they are. modulated. p

11. A signaling system including a circuit for transmitting a pluralityof carrier fre' quencies each modulated by a band of signals, means forsuppressing all of the carrier frequencies but one,.a circuit uponyWhich are impressed the unsuppressed carrier frequency and the band ofsignals modulatedV thereon as Well as the bands of'signals correspondingto the suppressed carrier frequencies, and means to derive the signalsmodulated on the unsuppressed carrier frequency.

12. A signaling system comprising a circuit for transmitting a pluralityof carrier waves each modulated by a band of signals, means forsuppressing all of the carrier Waves but one Without affecting any ofthe bands of signals modulated on said carrier Waves, and a detector forderiving the signals modulated on the unsuppresscd carrier Wave.

13. A signaling system comprising means for transmitting a plurality ofcarrier frequencies each modulated by bands of signals, means forsuppressing all of the carrier frequencies but one without affecting thebands of signals modulatedon any of said carrier frequencies, and meansto beat the unsuppressed carrier frequency and all of the bands ofsignals in order to derive the signals corresponding to the unsuppressedcarrier frequency.

14. A signaling system comprising means for transmitting a plurality ofcarrier waves cach modulated by a band of signals, means for suppressingall of the carrier Waves but one While freely transmitting the bands ofsignals previously modulated on all 0f said suppressed carrier Waves,and a demodulator to which are transmitted the unsuppressed carrier Waveand the bands of signals previously modulated on all of the carrierwaves, said demodulator deriving the original signals corresponding tothe unsuppressed carrier Wave, said demodulator rendering the remainingbands of signals substantially ineffect-ive 15. The combination of avacuum tube system coupled to a transmission circuit transmitting acarrier frequency and a side band corresponding to signals, and apiezo-electric device the natural period of vibration of which coincideswith the oscillations of carrier frequency, said piezo-electric devicesuppressing the oscillations of carrier frequency.

16. The combination of a circuit transmitting a band of signals andcurrent of a particular frequency which is not desired, and apiezo-electric device connected in shunt across said circuit, saiddevice having a natural period of vibration which coincides with theparticular undesired frequency.

17. The combination of a circuit transmitting current representing aband of signals and current of a particular frequency, and ashunt-connected piezo-electric device having a frequency of vibrationsubstantially the same as the particular frequency of the lattercurrent, said piezo-electric device by-passing quencies corresponding tothe particular frequencies to be discriminated against.

20. A filtering arrangement for separating a plurality of carrierfrequencies from the corresponding bands of signals transmittedtherewith, comprising a plurality of parallel connected piezo-electriccrystals of vibratory frequencies corresponding respectively to thevarious carrier frequencies.

2l. A transmission circuit transmitting a plurality of carrier Waves anda plurality of side bands of signals corresponding to the variouscarrier Waves, including a filtering arrangement for discriminatingagainst the carrier Waves, said arrangement consisting ofa plurality ofpiezo-electric crystals of different vibratory frequencies, saidvibratory frequencies respectively equaling the frequencies of saidcarrier waves.

In testimony whereof, I have signed my name to this specification this17th day of June, 1927.

CHARLES H. FETTER.

said latter current and exhibiting high attenuation to the formercurrent representing said band of signals.

18. An electrical filtering arrangement for discriminating against aplurality of different frequencies transmitted between an input circuitand an output circuit, comprising a plurality of piezo-electric crystalsof different frequency characteristics connected in parallelrelationship.

v 19. A filtering system for discriminating between currents ofparticular frequencies and all other currents, comprising a pluralit yof piezo-electric crystals of vibratory fre-

